You might check the output signal and aspect ratio of the output signal in your cable box menu settings. Those settings may have changed when it detected a HDMI cable rather than the component cables. HDMI sends a 1:1 pixel representation to the television, so it's quite possible that would have happened by default.
I purchased an LN55C610 last week, and hooked up my HD box with component cables.
I had the Picture Size settings set to 16:9 and all channels would fit all across the screen (HD and non HD)
Yesterday, I decided to change cables, hooking up a DVI > HDMI cable from my HD Cable box to the TV, and now the non hd channels will not display across the full screen as they did before in 16:9 (I have to use the "wide" setting, but now the bottom and top of the non hd channels are cut off)
In an effort to get my picture size settings back (in 16:9) I switched back to the component cables, and it still hasnt changed to the original settings.
Is there a reset I can do to restore how I was originally receiving all non hd channels in full screen 16:9 format?